Prop stopping vertically

RyanB

RyanB

Super Administrator
Management Council Member
PoA Supporter
Joined
Jul 21, 2010
Messages
16,524
Location
Chattanooga, TN
Display Name
Display name:
Ryan
I have read somewhere in the past that a prop that repeatedly stops vertical on a 4 cyl engine, is very likely indexed wrong to the crankshaft. Is this true?

Reason I ask is, one of the 172’s at the flight school seems to be this way. 9 times out of 10 it will stop vertical upon shutdown. It never occurred to me that it seems to happen more often on this particular airplane than on any of the others until I read about it, so it had me curious.
 
I was told, rightly or wrongly, that the prop should stop in roughly the same position each time. Which position is based IIRC on how it was "clocked."

so 9 of 10 in the same position sounds 'normal'.

I have 3 blades and it's clocked so one is vertical. Minimizes my manual prop manipulations for the tow bar.
 
Ryanb said:
I have read somewhere in the past that a prop that repeatedly stops vertical on a 4 cyl engine, is very likely indexed wrong to the crankshaft. Is this true?

Reason I ask is, one of the 172’s at the flight school seems to be this way. 9 times out of 10 it will stop vertical upon shutdown. It never occurred to me that it seems to happen more often on this particular airplane than on any of the others until I read about it, so it had me curious.
Click to expand...

In the 172 manual it tells the mechanic to rotate the crankshaft until #1 cylinder is at TDC, then hold the prop vertical and rotate it clockwise (as seen from the front of the airplane) until the prop's bolt holes line up with the crank's flange holes, and put it on. That will have the prop stop at the 10:00-4:00 position as seen from the front, and puts it in the best and safest position for hand-propping.
 
Last edited:
A 4 cylinder Lycoming with a 2-blade prop should stop with the prop at 10:00 and 4:00 when viewed from the front.
 
Every plane I have owned has stopped in numerous places, and my twin never stopped with its props in sync.
 
A 172M club I was in years ago majored their engine. When it came back it shook at crankshaft speed, to the point that they were considering opening everything up again until some sharp mechanic realized that the prop was oriented incorrectly.

I am a retired mechanical engineer experienced in vibration and engine designs, yet I don't understand why this is only a 172 problem, and not on other 4 cyl aircraft. But I fully accept that it is.
 
nrpetersen said:
A 172M club I was in years ago majored their engine. When it came back it shook at crankshaft speed, to the point that they were considering opening everything up again until some sharp mechanic realized that the prop was oriented incorrectly.

I am a retired mechanical engineer experienced in vibration and engine designs, yet I don't understand why this is only a 172 problem, and not on other 4 cyl aircraft. But I fully accept that it is.
Click to expand...

It could have been a stack up of balance tolerances of the various rotating components on that particular aircraft...ie; the balance of the crankshaft, prop, ring gear , and spinner assemblies all lined up to a bad combination. A common cure to a vibration problem on 2 blade props is to rotation the prop 180 degrees in the clock position.
 
Ryanb said:
Is this true?
Click to expand...
FWIW: Can't comment on the consistency of where a prop stops, but most props have a specific index point called out during installation which is usually stated in the MM--and not are all the same. Lycoming also has a SB out there that defines correct prop flange bushing locations to ensure proper prop indexing. Don't know about the hand-prop requirement, but it has been my understanding the prop indexing was to reduce engine/prop vibrations as mentioned above. And when troubleshooting errant prop/engine vibrations every shop/OEM I've talked to always questioned if the prop was installed in the correct position. So to reduce issues prior to performing a dynamic prop balance I would usually check prop position among other things prior to start.
 
Turn it by hand and see where the impulse coupler snaps. That's a pretty good indication of TDC. I wouldn't judge anything by where it stops when you turn it off.
 
James331 said:
What if you’re left handed (wrong handed) and it’s day light savings?
Click to expand...

Depends on what oil is in it. XC20W-50 with CamGuard will start on the first blade the first time, every time. AeroShell 100, without a flight plan, will require you to check with any traffic in the area, advise, get their advice, and the enter a right midfield downwind across the runway without any subsequent radio calls. None of this applies during Standard Time, so yank & bank til Spring!
 
First, a couple of humorous items.

One of the members of the flying club always turned it to 9 and 3 when he finished flying, IT LOOKS SO MUCH BETTER!

Airplane salesmen also turn the props to 9 and 3 so they can hang FOR SALE signs neatly on them.

Another member turned the prop to noon, TO KEEP THE BIRDS FROM PERCHING ON IT!



On a much more practical note, ALL of our 4 cylinder Cessna's stopped at 10 and 4, both Lycoming and Continental. In this position you have the best angle for hand propping the engine strongly and safely. I was taught proper technique half a century ago by an experienced CFI, and it is too difficult to accurately describe to attempt in a written form.

I have propped 4 cylinders up to 240 HP, and 6 cylinders to about 280 HP. Oddly, taildraggers are easier to safely prop than trikes.

6 cylinder engines stop at 3 different angles, 120 degrees apart, and the only position they should be propped from is 10 & 4. If you fail to get a start from the first try, switch off, turn carefully to 10 again, switch on, and try another time.

Our Lycoming has a mark on the crank flange which aligns with a mark on the prop, to prevent 180 out installation, which is important to us, as we have had the engine dynamically balanced.

I have assisted mechanics who removed and replaced props for us about a dozen times, and they usually put a mark on hub and prop with a magic marker or equivalent, to be sure they put them back on as found.
 
JohnAJohnson said:
Not all of them. Two-Bladed Cardinals (IO360 or O360) stop at nine and three.
Click to expand...
CS props often have a recessed bolt hole at one spot on the mating face of the hub, and a larger bolt dowel in the flange fits into it to index the prop. There would have been reasons of balance considered in that case, and the 9 - 3 stopped prop is the result.
 
DesertNomad said:
My 6-cyl Dakota stops all different places.
Click to expand...
The prop wants to stop in the easiest position for the crankshaft, which is determined by two of the cylinders with pressure in them. There will be one coming up on TDC on its pressure stroke and another going down on its power stroke, before its exhaust valve is open, so the crank will want to rest where the pressures in those two cylinders are about equal. On a four-banger there are two positions that look like one because they're directly opposite each other. The six has three positions, so a two-blade prop will pick one of those three, and a three-blade prop will look like it has one position.

A hot engine sometimes stops fairly abruptly when shut down and all the above is just theory. Clearances are tight and oil is really thin when things are hot enough. Try moving that prop a few minutes after shutdown (don't get bit if it fires!) and then try again once it's cooled off. Big difference in friction.
 
Mine stops at 7:42 and 1:42, but I think it must be Zulu time or something because it’s never the right time.
 
Dan Thomas said:
In the 172 manual it tells the mechanic to rotate the crankshaft until #1 cylinder is at TDC, then hold the prop vertical and rotate it clockwise (as seen from the front of the airplane) until the prop's bolt holes line up with the crank's flange holes, and put it on. That will have the prop stop at the 10:00-4:00 position as seen from the front, and puts it in the best and safest position for hand-propping.
Click to expand...
is this the same for 4cyl Lyc's and 6cyl Conti's or.....?
asking cause I have an older 172 with the 6cyl Conti
 
On the IO-550 on a Cirrus, the prop stops at the same position each time.
 
John asked:

is this the same for 4cyl Lyc's and 6cyl Conti's or.....?
asking cause I have an older 172 with the 6cyl Conti

The Cessna 6 cylinder Continental should be at 10 when at rest, or 120, 240 degrees more.

The club had a 56 172, and in cold weather, I often hand propped it due to weak battery.

The description of Switch off, rotate until 10 o'clock, was specifically that plane and engine.

Those are very smooth running engines, and except for their famous tendency for carb ice, my wife and I enjoyed it for many cross country miles.
 
Three bladed props have the same transverse moments-of-inertia no matter what plane they are being shaken. A two blade prop moment-of-inertia will be grossly different in the two transverse axes. All this is is critical as a 4 cylinder opposed engine inherently swings in a yaw axis at twice crankshaft frequency.

A 6 cylinder engine doesn't inherently swing. It is all a balance thing.
 
If the average 4 cyl Lycoming has a prop vibration the first step is to reclock the prop 180*. Still 10 and 4 but with blade position reversed. Mine have benefitted from it. My CS prop also stops at 10 and 4, and while the prop flange does have bushings prop can be installed with any bolt in that position. I’ve re-clocked that prop, too. Always 10 and 4.
 
nrpetersen said:
All this is is critical as a 4 cylinder opposed engine inherently swings in a yaw axis at twice crankshaft frequency.

A 6 cylinder engine doesn't inherently swing. It is all a balance thing.
Click to expand...

Are you referring to what is sometimes called a "rocking couple"? I thought this was caused by the opposing cylinders not being in the same geometric plane and thus not cancelling each other out perfectly. Why would this not be true of an opposed six cylinder also?
 
Are you referring to what is sometimes called a "rocking couple"? I thought this was caused by the opposing cylinders not being in the same geometric plane and thus not cancelling each other out perfectly. Why would this not be true of an opposed six cylinder also?
Yes - You are correct in that is caused by the cylinder pairs being offset rather than in a perfect H layout. I once thought the second harmonic cancels in a 6 cyl engine with L-R cylinder offset, but no way could I prove that! Now I'm not even so sure.... Any other engine geeks or nerds out there?
 
Clip4 said:
On the IO-550 on a Cirrus, the prop stops at the same position each time.
Click to expand...
Yup, as I said in post #26: The six has three positions, so a two-blade prop will pick one of those three, and a three-blade prop will look like it has one position.
 
A weak cylinder can also contribute to the prop not stopping in the same position.
 
geezer said:
One of the members of the flying club always turned it to 9 and 3 when he finished flying, IT LOOKS SO MUCH BETTER!

Airplane salesmen also turn the props to 9 and 3 so they can hang FOR SALE signs neatly on them.

Another member turned the prop to noon, TO KEEP THE BIRDS FROM PERCHING ON IT!
Click to expand...

And mechanics will position the prop at 12 and 6 to keep from walking into it.
 
You must log in or register to reply here.
Back
Top